Device-to-Device (D2D) communication is a component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards suite such as WiFi Direct. These systems operate in unlicensed spectrum.
Recently, D2D communication—alternatively referred to as “proximity based services (ProSe) Direct Communication”—has been proposed as an underlay to cellular networks to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, such D2D communication, or ProSe Direct Communication, shares the same spectrum as the cellular system by, for example, reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.
When sending data during D2D communication, the transmission mode may be either unicast, multicast, or broadcast. In the unicast mode, a specific User Equipment (UE) is the receiver. In the multicast mode, which can also be denoted “group-cast,” a group of UEs are receivers. In the broadcast mode, all UEs are receivers.
With connectionless D2D communication, data can be sent from one device to another device without prior arrangement, thereby reducing the overhead and increasing the communication capacity, which is crucial in emergency situations. The source device transmits data to one (unicast) or more (multicast/group-cast/broadcast) other devices, without first ensuring that the recipients are available and ready to receive the data. Connectionless communication may be used for one-to-one or one-to-many communication, but it is particularly effective for multicast and broadcast transmissions and thus well-suited for broadcast and group communications. The connectionless communication may be realized, e.g., via physical layer (PHY) unicast/multicast/group-cast/broadcast transmissions. With PHY broadcast transmissions, the transmissions may still be turned into unicast/group-cast/multicast at higher layers. For example, in the Medium Access Control (MAC) layer, multicast or even unicast addresses may be used. Alternatively, if using broadcast on both PHY and MAC, multicast or unicast Internet Protocol (IP) addresses may be used at the IP layer.
When a UE is in network coverage, any D2D communication is controlled by the network nodes (such as the enhanced or evolved Node B (eNB)). Since the radio resources in a cell (especially for the uplink resources) are shared between traditional cellular communication and D2D communication, the eNB should also divide and assign the radio resources in case of D2D communication in case the UEs are in coverage. In Third Generation Partnership Project (3GPP) Release 12 (Rel-12), the ProSe UE Information message has been introduced as part of the Radio Resource Control (RRC) protocol. This message is used whenever the UE needs to inform the eNB about a need for ProSe communication or ProSe discovery. For communication, the ProSe UE Information message contains a list of ProSe destinations, and an index associated to each of these ProSe destinations. In case of multicast communication, a ProSe destination is a ProSe Layer 2 Group identity and, for unicast communication, a ProSe destination is a ProSe UE Identity. The index may later be used as a 4-bit short reference to a given group or unicast destination, e.g. as used in the MAC Buffer Status Report when transmitting data to the destination.
Moreover, a given unicast traffic session between two UEs may use either a direct communication path or an infrastructure communication path. When using the direct communication path, the data is transmitted directly between the UEs using D2D communication. On the other hand, when using the infrastructure communication path, the data is instead transmitted via the network nodes. The latter case is only available when both UEs are in coverage of the network.
A service continuity switch is the procedure to move a user traffic session from the direct communication path to the infrastructure communication path, or vice versa. In 3GPP Release 13 (Rel-13), service continuity switching will likely be included. By “direct communication path,” it is meant that the transmitted packets use D2D communication (sidelink) channels. By “infrastructure communication path,” it is meant that the packets use the non-D2D, legacy, physical (uplink and downlink) channels and also that the packets are transmitted over an Evolved Packet System (EPS) bearer, which is effectively a tunnel between the UE and the Packet Data Network (PDN) Gateway (P-GW) network node.
ProSe communication is introduced into 3GPP at Rel-12. One potential topic in Rel-13 would be the service continuity, i.e., service continuity between infrastructure and ProSe Direct Communication paths, where one of the scenarios is shown FIG. 1. A user traffic session is kept even when a UE goes between in coverage and out of coverage. In this scenario, the mobility is limited to one UE (UE1), and the other UE (UE2) acts as the relay between UE1 and the network (i.e., the eNB). In this scenario UE2 is defined as the “relay UE” of UE1, and UE1 is the “remote UE” of UE2.
Relays are a key feature of Long Term Evolution (LTE) Advanced (LTE-Advanced) introduced in Release 10 (Rel-10) of the LTE specifications. Early relays, in the form of repeaters, are present in legacy radio interface technologies such as the Universal Mobile Telecommunications System (UMTS) and Release 8 (Rel-8) of LTE. Relays are used to improve coverage in zones where the traffic is too light to justify the deployment of a base station or where there is no easy backhaul network access, such as road segments in rural areas. Thus, the relay deployment is traditionally done with the purpose of coverage extension, where the coverage is measured based on the desired signal received power. In LTE, the power of the received desired signal is measured using reference signals on uplink/downlink. This measurement is known as Reference Signal Received Power (RSRP), which is discussed below.
RSRP is the average power of Resource Elements (REs) that carry cell specific Reference Signals (RSs) over the entire bandwidth. RSRP is only measured on the symbols carrying RS. The typical range of RSRP is around −44 to −130 decibel-milliwatts (dBm). This measurement is used in RRC Idle/Connected, Cell Reselection/Selection, handover scenarios. Since this measures only the reference power, it can be said that this is the strength of the wanted signal but it does not give any information about signal quality. RSRP gives us the signal strength of the desired signal, not the quality of the signal. For quality of the signal information another parameter called Reference Signal Received Quality (RSRQ) is used in some cases.
Now, consider the UE-Network relaying scenario where a relay is itself a UE (relay UE) that has the capability to provide service to another UE (remote UE). In the following, relay selection and re-selection is described in view of the UE-Network relay scenario where the relay is a relay UE. Relay selection and re-selection is explained with respect to an example shown in FIG. 2 where there are two candidate relay UEs (UE1 and UE2) and an out-of-coverage, remote UE (UE3). In the case of relay selection, the remote UE (UE3) wants to reach the eNB by attaching itself to one of the relay UEs (UE1 and UE2) that can provide a desired service(s) with desired reliability. The question is under what rules/principles a relay should be selected. In the case of relay reselection, the remote UE (UE3) is already attached to one relay. However, if for some reason the serving relay cannot provide good service any longer, then another relay can be selected.
Existing technologies for relay selection/reselection are based on measurements of link quality between nodes involved in the multi-hop route. Such measurements may be reported to other nodes or to the eNB in order to assist the routing decision.